How do I rotate the bones?
Go into edit mode.
Select the tip of the second bone and snap the cursor to it. &#8679; ShiftS
Select the root of the first bone and extrude a new helper bone from it. E
Select the tip of the helper bone and snap it to the cursor. &#8679; ShiftS





Select the helper bone.
Choose the normal orientation for transformations.
Create a custom transform orientation from the normal orientation.
Delete the helper bone. (We have the custom orientation from the root of the first bone to the tip of the second bone, and that is all that we need.) X
Choose the 3D Cursor as Pivot Point. . / numpadperiod
Select the root of the second bone and scale it to 0 on the custom XZ axis, by pressing S[&#8679; ShiftY]0. (Shift has to be held while Y is pressed, this constrain scaling along the XZ axis of the custom transform orientation.)





Copy paste the roll property from one bone to the other.




This answer expands on how the custom transform orientation works.